Thanks to Panj's great publicity campaign in the English Only forums, I have found my way here to make a small contribution.

- stubborn as a mule
- bull-headedBoth mean to be very stubborn, not wanting to give in to another's point of view.

-"Well, I'll be a monkey's uncle!"This is more of a regional colloquialism or expression, which expresses surprise and/or delight!

- cute as a bug (Do insects count as "animals?" for our purposes here?)Used to describe someone or something small and diminuitive that is considered as cute. Babies are often described as "cute as a bug."

- snug as a bug in a rug
The feeling of being warm and cozy under the covers. Often expressed by children after they have been "tucked in" to bed at night. (Not to be confused with the BE version of "tuck in" which means to eat! )

- the cat's meow
- the cat's pajamas
- the bee's kneesThese are a bit archaic, but are still heard from time to time to describe something as excellent. Alanis Morrisette has a song in which she describes her new love as the "knees of my bees."

- I don't give a rat's assI don't care. This obviously has a more vulgar use and is primarily used among people who are very familiar with one another, mostly in heated discussions.

- quiet as a mouseThis describes people who are very quiet and hardly make a "peep."

Edit: Thought of two, three, okay, so four more...(below)

- when pigs fly
Used to express an idea of incredulity about a future event.
(Similar to "when hell freezes over.")

- until the cows come homeThis might be more regional. It is used to describe a long period of waiting. MOOOOO!!!

- gentle as a lamb- wouldn't hurt/harm a flyUsed to describe someone who is particularly gentle and non-threatening.

avoir une fièvre de cheval: to have a horse fever (to run a high fever)
le jour où les poules auront des dents:the day the chicken have teeth. (it is unlikely to hapen soon)
avoir le cafard: to have a cockroach (to have the blues)
une hirondelle ne fait pas le printemps: one swallow does not make spring (seing a swallow does not mean that nice weather is garanteed)
ça na casse pas trois pattes à un canard: it does not break three legs of a duck (it is nothing out of the ordinary)
un mouton à cinq pattes: a five legged sheep (a hard to find object that conforms to lots of requirements) for instance if someone wants an economical car with lots of inside room and comfort, easy to park maintain etc....and does not want to spen more than 2000euros, one might say:"il n'est pas près de trouver son mouton à cinq pattes!"
vexé comme un rat mort en couches: vexed as a rat that died when giving birth (highly vexed)